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<th width="60%" align="center">Chapter 29. Multi-threaded programs</th>
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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-multithread-example"></a>Example</h2></div></div></div>
<p>
This is an example program with two threads, one GUI thread, like in all
<span class="application">gtkmm</span> programs, and one worker thread. The worker thread is created when you
press the <code class="literal">Start work</code> button. It is deleted when the work is
finished, when you press the <code class="literal">Stop work</code> button, or when you
press the <code class="literal">Quit</code> button.
</p>
<p>
A <code class="classname">Glib::Dispatcher</code> is used for sending notifications
from the worker thread to the GUI thread. The <code class="classname">ExampleWorker</code>
class contains data which is accessed by both threads. This data is protected
by a <code class="classname">std::mutex</code>.
Only the GUI thread updates the GUI.
</p>
<div class="figure">
<a name="figure-multithread"></a><p class="title"><b>Figure 29.1. Multi-Threaded Program</b></p>
<div class="figure-contents"><div class="screenshot"><div><img src="figures/multithread.png" alt="Multi-Threaded Program"></div></div></div>
</div>
<br class="figure-break"><p><a class="ulink" href="http://git.gnome.org/browse/gtkmm-documentation/tree/examples/book/multithread?h=gtkmm-3-24" target="_top">Source Code</a></p>
<p>File: <code class="filename">examplewindow.h</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H
#include <gtkmm.h>
#include "exampleworker.h"
class ExampleWindow : public Gtk::Window
{
public:
ExampleWindow();
// Called from the worker thread.
void notify();
private:
// Signal handlers.
void on_start_button_clicked();
void on_stop_button_clicked();
void on_quit_button_clicked();
void update_start_stop_buttons();
void update_widgets();
// Dispatcher handler.
void on_notification_from_worker_thread();
// Member data.
Gtk::Box m_VBox;
Gtk::ButtonBox m_ButtonBox;
Gtk::Button m_ButtonStart;
Gtk::Button m_ButtonStop;
Gtk::Button m_ButtonQuit;
Gtk::ProgressBar m_ProgressBar;
Gtk::ScrolledWindow m_ScrolledWindow;
Gtk::TextView m_TextView;
Glib::Dispatcher m_Dispatcher;
ExampleWorker m_Worker;
std::thread* m_WorkerThread;
};
#endif // GTKMM_EXAMPLEWINDOW_H
</pre>
<p>File: <code class="filename">exampleworker.h</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#ifndef GTKMM_EXAMPLEWORKER_H
#define GTKMM_EXAMPLEWORKER_H
#include <gtkmm.h>
#include <thread>
#include <mutex>
class ExampleWindow;
class ExampleWorker
{
public:
ExampleWorker();
// Thread function.
void do_work(ExampleWindow* caller);
void get_data(double* fraction_done, Glib::ustring* message) const;
void stop_work();
bool has_stopped() const;
private:
// Synchronizes access to member data.
mutable std::mutex m_Mutex;
// Data used by both GUI thread and worker thread.
bool m_shall_stop;
bool m_has_stopped;
double m_fraction_done;
Glib::ustring m_message;
};
#endif // GTKMM_EXAMPLEWORKER_H
</pre>
<p>File: <code class="filename">main.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "examplewindow.h"
#include <gtkmm/application.h>
int main (int argc, char* argv[])
{
auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
ExampleWindow window;
//Shows the window and returns when it is closed.
return app->run(window);
}
</pre>
<p>File: <code class="filename">exampleworker.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "exampleworker.h"
#include "examplewindow.h"
#include <sstream>
#include <chrono>
ExampleWorker::ExampleWorker() :
m_Mutex(),
m_shall_stop(false),
m_has_stopped(false),
m_fraction_done(0.0),
m_message()
{
}
// Accesses to these data are synchronized by a mutex.
// Some microseconds can be saved by getting all data at once, instead of having
// separate get_fraction_done() and get_message() methods.
void ExampleWorker::get_data(double* fraction_done, Glib::ustring* message) const
{
std::lock_guard<std::mutex> lock(m_Mutex);
if (fraction_done)
*fraction_done = m_fraction_done;
if (message)
*message = m_message;
}
void ExampleWorker::stop_work()
{
std::lock_guard<std::mutex> lock(m_Mutex);
m_shall_stop = true;
}
bool ExampleWorker::has_stopped() const
{
std::lock_guard<std::mutex> lock(m_Mutex);
return m_has_stopped;
}
void ExampleWorker::do_work(ExampleWindow* caller)
{
{
std::lock_guard<std::mutex> lock(m_Mutex);
m_has_stopped = false;
m_fraction_done = 0.0;
m_message = "";
} // The mutex is unlocked here by lock's destructor.
// Simulate a long calculation.
for (int i = 0; ; ++i) // do until break
{
std::this_thread::sleep_for(std::chrono::milliseconds(250));
{
std::lock_guard<std::mutex> lock(m_Mutex);
m_fraction_done += 0.01;
if (i % 4 == 3)
{
std::ostringstream ostr;
ostr << (m_fraction_done * 100.0) << "% done\n";
m_message += ostr.str();
}
if (m_fraction_done >= 1.0)
{
m_message += "Finished";
break;
}
if (m_shall_stop)
{
m_message += "Stopped";
break;
}
}
caller->notify();
}
{
std::lock_guard<std::mutex> lock(m_Mutex);
m_shall_stop = false;
m_has_stopped = true;
}
caller->notify();
}
</pre>
<p>File: <code class="filename">examplewindow.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "examplewindow.h"
#include <iostream>
ExampleWindow::ExampleWindow() :
m_VBox(Gtk::ORIENTATION_VERTICAL, 5),
m_ButtonBox(Gtk::ORIENTATION_HORIZONTAL),
m_ButtonStart("Start work"),
m_ButtonStop("Stop work"),
m_ButtonQuit("_Quit", /* mnemonic= */ true),
m_ProgressBar(),
m_ScrolledWindow(),
m_TextView(),
m_Dispatcher(),
m_Worker(),
m_WorkerThread(nullptr)
{
set_title("Multi-threaded example");
set_border_width(5);
set_default_size(300, 300);
add(m_VBox);
// Add the ProgressBar.
m_VBox.pack_start(m_ProgressBar, Gtk::PACK_SHRINK);
m_ProgressBar.set_text("Fraction done");
m_ProgressBar.set_show_text();
// Add the TextView, inside a ScrolledWindow.
m_ScrolledWindow.add(m_TextView);
// Only show the scrollbars when they are necessary.
m_ScrolledWindow.set_policy(Gtk::POLICY_AUTOMATIC, Gtk::POLICY_AUTOMATIC);
m_VBox.pack_start(m_ScrolledWindow);
m_TextView.set_editable(false);
// Add the buttons to the ButtonBox.
m_VBox.pack_start(m_ButtonBox, Gtk::PACK_SHRINK);
m_ButtonBox.pack_start(m_ButtonStart, Gtk::PACK_SHRINK);
m_ButtonBox.pack_start(m_ButtonStop, Gtk::PACK_SHRINK);
m_ButtonBox.pack_start(m_ButtonQuit, Gtk::PACK_SHRINK);
m_ButtonBox.set_border_width(5);
m_ButtonBox.set_spacing(5);
m_ButtonBox.set_layout(Gtk::BUTTONBOX_END);
// Connect the signal handlers to the buttons.
m_ButtonStart.signal_clicked().connect(sigc::mem_fun(*this, &ExampleWindow::on_start_button_clicked));
m_ButtonStop.signal_clicked().connect(sigc::mem_fun(*this, &ExampleWindow::on_stop_button_clicked));
m_ButtonQuit.signal_clicked().connect(sigc::mem_fun(*this, &ExampleWindow::on_quit_button_clicked));
// Connect the handler to the dispatcher.
m_Dispatcher.connect(sigc::mem_fun(*this, &ExampleWindow::on_notification_from_worker_thread));
// Create a text buffer mark for use in update_widgets().
auto buffer = m_TextView.get_buffer();
buffer->create_mark("last_line", buffer->end(), /* left_gravity= */ true);
update_start_stop_buttons();
show_all_children();
}
void ExampleWindow::on_start_button_clicked()
{
if (m_WorkerThread)
{
std::cout << "Can't start a worker thread while another one is running." << std::endl;
}
else
{
// Start a new worker thread.
m_WorkerThread = new std::thread(
[this]
{
m_Worker.do_work(this);
});
}
update_start_stop_buttons();
}
void ExampleWindow::on_stop_button_clicked()
{
if (!m_WorkerThread)
{
std::cout << "Can't stop a worker thread. None is running." << std::endl;
}
else
{
// Order the worker thread to stop.
m_Worker.stop_work();
m_ButtonStop.set_sensitive(false);
}
}
void ExampleWindow::update_start_stop_buttons()
{
const bool thread_is_running = m_WorkerThread != nullptr;
m_ButtonStart.set_sensitive(!thread_is_running);
m_ButtonStop.set_sensitive(thread_is_running);
}
void ExampleWindow::update_widgets()
{
double fraction_done;
Glib::ustring message_from_worker_thread;
m_Worker.get_data(&fraction_done, &message_from_worker_thread);
m_ProgressBar.set_fraction(fraction_done);
if (message_from_worker_thread != m_TextView.get_buffer()->get_text())
{
auto buffer = m_TextView.get_buffer();
buffer->set_text(message_from_worker_thread);
// Scroll the last inserted line into view. That's somewhat complicated.
Gtk::TextIter iter = buffer->end();
iter.set_line_offset(0); // Beginning of last line
auto mark = buffer->get_mark("last_line");
buffer->move_mark(mark, iter);
m_TextView.scroll_to(mark);
// TextView::scroll_to(iter) is not perfect.
// We do need a TextMark to always get the last line into view.
}
}
void ExampleWindow::on_quit_button_clicked()
{
if (m_WorkerThread)
{
// Order the worker thread to stop and wait for it to stop.
m_Worker.stop_work();
if (m_WorkerThread->joinable())
m_WorkerThread->join();
}
hide();
}
// notify() is called from ExampleWorker::do_work(). It is executed in the worker
// thread. It triggers a call to on_notification_from_worker_thread(), which is
// executed in the GUI thread.
void ExampleWindow::notify()
{
m_Dispatcher.emit();
}
void ExampleWindow::on_notification_from_worker_thread()
{
if (m_WorkerThread && m_Worker.has_stopped())
{
// Work is done.
if (m_WorkerThread->joinable())
m_WorkerThread->join();
delete m_WorkerThread;
m_WorkerThread = nullptr;
update_start_stop_buttons();
}
update_widgets();
}
</pre>
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